This invention refers to a process for starting and operating a brushless direct current motor and the electronic circuit thereof, specifically that of the type which detects the rotor position through induced voltages.
Brushless direct current motors are of interest in applications requiring high reliability and high efficiency such as household appliances, pumps and ventilators. Said motors include a stator having coils, a permanent magnet rotor, an inverter feeding current to the stator coils, a rotor position sensor which gives information on the time and period each coil assembly has to be kept powered and a central control which processes current, speed and position of the rotor information and sends command pulses to the inverter. In some applications requiring a simple and a strong structure of the motor, without using rotor position detection is effected by observing the induced voltages on the motor windings. It is necessary in these types of motor to use a special technique to start the motor, since when in a static situation without rotor movement, voltages are not induced to the coils, and the rotor position is unknown.
Several techniques for starting said motors are known. One is to detect the rotor position when it is stopped by means of a secondary sensor which is only used for motor starting. In this way, it is possible to decide upon which motor phases have to be powered in order that torque is produced in the desired direction, thereby starting the motor.
A problem with this solution is the need of an additional sensor being installed inside the motor plus the connections between such sensor and the control circuit, which increase the assembly complexity and cost.
Another solution, as described in the Japanese document, Kokai No. 55,5035 A, is to cause the inverter to supply current to the stator coils at a correct sequence thereby gradually increasing the frequency. This ignores the induced voltages being used to detect the rotor position, thereby causing the rotor to accelerate until reaching a speed at which the voltages induced in the stator coils are enough to cause the rotor position be detected. During this first step, the motor operates as a synchronous motor. Once this condition is reached, the control circuit observes the information given by the position sensor through the voltages induced to the stator, and the acceleration and the normal operation of the motor is continued and now it is operating in a self-controlled way.
One advantage of this method is that, depending upon the current value applied to the stator during the start in a synchronous form, synchronism loss can occur, resulting in stopping of the rotor or in some cases the motor will start in the reverse direction opposed to the desired direction. A further drawback is the vibration level which is relatively high during synchronous operation. The implementation of this solution can be complex, mainly if the method for determining the condition for the commutation from synchronous form to self-controlled form is based upon the observation of the phase between the internally generated synchronous signals and the signals given by the position sensor through induced voltages. This already known solution is implemented with the help of a microprocessor.